A drill is a cutting tool which can be used for piercing steel products and the like. For example, the structure of a twist drill is shown in FIG. 1. The twist drill is formed by a cutting portion 1 which is applied to piercing, and a shank portion 2 which is not concerned in cutting but is mainly used for discharging chips and to be clamped in a chuck on the shaft of a drilling machine.
In general, materials for drills are high-speed steel (rapid tool steel) and cemented carbide. The high-speed steel, which has an excellent toughness but is inferior in its wear resistance, is improper for high-speed cutting. On the other hand, the cemented carbide, which has an excellent wear resistance and accuracy characteristics as a tool but is quite brittle and hence may break when used in a machine tool having low rigidity, for example.
In order to improve drills there has been considered a drill structure with a coating of hard TiN on the cutting portion made of high-speed steel, or a drill structure with a cutting portion made of cemented carbide and brazing the same. There have further been devised different connecting structures e.g. by brazing different materials of cemented carbide (P30 and D30) for improving the wear resistance and toughness, (Japanese Utility Model Laying-Open Gazette No. 58-143115) or a structure of metallurgically attaining an integration or connection (Japanese Utility Model Laying-Open Gazette No. 62-46489), or a structure that takes into account the difference between the characteristics required for a central portion and an outer peripheral portion of a drill and preparing the central portion and the outer peripheral portion from different materials of cemented carbide in a double structure (Japanese Patent Laying-Open Gazette No. 62-218010), or a method of forming such a double structure by injection molding (Japanese Patent Laying-Open Gazette No. 63-38501 or 38502). Further, there is a structure of forming the material for a drill as a cermet, in order to improve adhesion resistance of the drill (Japanese Patent Laying-Open Gazette No. 62-292307).
A cutting portion and a shank portion of a drill are exposed to different loaded conditions in a drilling operation. Therefore, characteristics required for the respective portions of the drill are different from each other. For example, wear resistance, adhesion resistance and the like are required for a cutting edge of the cutting portion, while toughness for maintaining the strength of the tool is required for the shank portion. Further, different cutting edge characteristics are required for different portions along the cutting edge since cutting speeds are different at the center and at the outer peripheral portions of a drill cutting edge.
In a drill whose cutting edge is coated in order to satisfy these different requirements at least a coating layer of a front flank side is inevitably removed when resharpening the drill as is performed in general, whereby most of the coating effect is lost. A drill having such a structure that cemented carbide is brazed to its cutting portion cannot be used for deep hole drilling of a hard to cut material since brazing itself is a method which is essentially inferior in thermal strength and mechanical strength. Further, when cemented carbide is brought into a coarse grain state or a high binder phase state in order to improve the toughness of the shank portion of a drill, the strength of the material is reduced or the elastic limit of distortion is reduced, and hence the shank portion is undesirably broken during drilling, due to vibration of a workpiece, unstable rotation of a drive shaft and the like. Also, the cost of a drill having a large diameter and a long length for drilling deep holes, depends on the drill structure. Drills entirely made of cemented carbide are problematically high-priced both in alloy costs and processing costs.
In order to solve the aforementioned problems, there has recently been devised the so-called throw-away tipped drill, whose cutting portion and shank portion are separately manufactured and mechanically connected to be separable from each other, thereby enabling the use of materials having the characteristics required for the respective drill portions.
Conventional throw-away tipped drills include those shown in FIGS. 2 to 5, for example. FIGS. 2 to 4 show a two-flute throw-away tipped drill, wherein the tips 11a and 11b are fixed to the outer peripheral portion and the inner peripheral portion of a forward end of a shank portion 12 by respective screw connections. On the other hand, FIG. 5 shows a typical example of a one-flute throw-away tipped drill, wherein a cutting portion 21 is fitted in a shank portion 22 as shown by an arrow, and fixed to a screw hole 24 by a screw 23. A coolant is directly supplied from a coolant supply hole 25 to the cutting edge of the cutting portion 21. A chip breaker 26 for chip parting control is formed on the cutting edge of the cutting portion 21.
As another type of a one-flute throw-away tipped drill, there is a spade drill which is defined in JIS B0171-1609. The spade drill is common to each of the aforementioned throw-away tipped drills in that a plate-type cutting portion is fixed to the forward end of a shank portion by screwing.
The aforementioned conventional throw-away tipped drills, whose cutting portions and shank portions are formed of different members, can solve the various problems of the integral drill whose cutting portion is integrated with and made of the same material as its shank portion and of the brazed drill formed by brazing cemented carbide to its cutting portion.
However, each of these throw-away tipped drills, whose cutting portions and shank portions are connected with each other by screwing, causes the following problems:
(1) When the screw hole is increased in size, the tip strength is reduced. PA1 (2) In order to reduce the size of the screw hole, it is necessary to use a small screw, and hence an allowable load for the screw is reduced to inevitably lower the strength of the connection between the cutting portion and the shank portion.
(3) Further, a small screw is inferior in its workability in assembling. In the conventional throw-away tipped drill, consequently, the cutting portion and the shank portion have been easily broken during drilling operations due to insufficiency in strength of the cutting portion and insufficiency in the connection strength between the cutting portion and the shank portion in the case of a small-diameter drill of not more than 20 mm in diameter.